1. Technical Field
The present invention relates generally to a knock strategy for an internal combustion engine and more particularly to a knock strategy which detects engine knock on an individual cylinder basis.
2. Discussion
A conventional four-cycle internal combustion engine typically includes at least four pistons located within corresponding piston cylinders. Each of these pistons has four associated strokes during the engine combustion cycle: a fuel intake stroke, a fuel compression stroke, an expansion stroke in which the piston rotates the engine crankshaft, and an exhaust stroke in which the burned gas and associated byproducts are exhausted from the cylinder. At some point subsequent to the piston intake stroke and prior to the piston reaching top dead center (TDC) on the compression stroke, a spark plug associated with the cylinder fires a spark to initiate the combustion that drives the piston in its expansion stroke. The spark advance, or timing, of the spark determines the phasing of the burn for the fuel/air mixture in the cylinder, and thus directly affects engine performance.
The above-mentioned combination of load, spark advance and fuel/air mixture determines the temperature of the residual, which is the burned gas that is retained in the cylinder. Hence, the temperature of the residual gas thereby directly affects subsequent combustion events and the occurrence of knock. Ideally, the combustion of the fuel/air mixture within the cylinder is caused by a uniform flame front that progresses across the cylinder over a finite period of time. However, depending upon the temperature of the residual gas, engine load and ratio of fuel in the fuel/air mixture, the flame front within a cylinder may progress in a non-uniform manner, causing the gases within the cylinder to expand through several localized explosions, collectively known as knock, rather than in one uniform and sustained explosion. The localized explosions cause the pressure of the gases within the cylinder to oscillate, sending vibrations through the cylinder block, piston, connecting rod and crankshaft.
As is known in the art, a "well placed" sensor which is tuned to the correct frequency so as to maximize the signal-to-noise ratio can be placed in the engine to sense the vibrations created by knock. However, the inherent flaw of the "well-placed" sensor strategy is that the magnitude of the vibrations sensed by the sensor vary as a function of the distance from the cylinder which is generating the vibrations (i.e., knocking). Therefore, assuming knock is occurring in two cylinders at an equivalent magnitude and that the sensor is closer to one of these cylinders than the other, the sensor will detect two levels of vibration: a higher magnitude series of vibrations from the cylinder closest the sensor and a lower magnitude series of vibrations from the more distant cylinder.
Several strategies have been developed which employ a "well-placed" sensor and a single threshold vibration level to detect the occurrence of knock in any of the engine cylinders (i.e., globally). Typically, this threshold vibration level coincides with a knocking condition in the cylinder furthest the sensor. Consequently, cylinders closer to the sensor which are creating vibrations but not suffering from a knocking condition could inadvertently cause premature deployment of a knock control method. Deployment of these knock control methods immediately reduces the temperature in the cylinders by reducing the temperature of the residual. In accomplishing this reduction in temperature, the knock control methods must naturally effect the efficiency with which the fuel/air mixture combusts. Therefore, premature detection of knocking unnecessarily effects fuel economy in an adverse manner.
Other variations have been developed in an effort to overcome the problems associated with global monitoring with a single "well-placed" sensor. One alternative has been to tailor the knock detection strategy to the vibrations of a single cylinder which has been statistically determined through empirical testing to be more prone to knock than the other cylinders. While this strategy has been employed with favorable results, recent improvements in the delivery of air and fuel to the individual engine cylinders have created engines with very little cylinder-to-cylinder variation. Consequently, these improvements have eliminated the ability to reliably use any one cylinder as an indicator of whether knock is occurring in an engine. Stated another way, while an individual engine may have a specific cylinder which is more prone to knock, the location of this cylinder varies from engine to engine and as such, no one location can produce reliable results under this method.
Another strategy that has been developed employs a "well placed" sensor to globally monitor vibrations, identify a cylinder which is experiencing a knocking condition and direct any knock control activities to the cylinder experiencing knock, exclusively. While this type of strategy provides the best capabilities to detect and control knock while minimizing the adverse effects on fuel economy, there are several significant drawbacks to this strategy. One such drawback is the requirement to precisely control fuel and air at each individual cylinder. This requirement often requires additional controls equipment and software, as well as complicating the assembly and servicing of the engine. For example, where an engine manufacturer employs recirculated exhaust gases to reduce the amount of oxygen in the intake air, several control valves corresponding to each specific cylinder must be incorporated into the intake manifold ports. These per-cylinder valves ensure precise control of the air mixture flowing into the knocking cylinder as well as prevent recirculated exhaust gases from being directed to an adjacent non-knocking cylinder. Naturally, the costs associated with the substantial addition of controls equipment is quite significant, both in terms of direct costs (e.g., piece costs, assembly labor) and indirect costs (e.g., warranty, ability to service the engine).
Consequently, there remains a need in the art for an improved knock strategy which globally detects knock and provides the capability to control knock in a cost-effective manner.